I couldn't read this thread and not respond.
Please don't worry about this, and don't stop him spinning or view it as 'negative' behavious. He is still very young, so even if it is an indication of something like asd he is too young for diagnosis and stopping him spinning won't make him better anyway. He obviously needs to spin, for whatever reason.
From a very young age DS has been obsessed with spinning, so much so that for the length of a washing cycle he could sit mesmerised in front of the washing machine at the age of 1yr. He still does and he's 6 now.
I realised quite quickly that spinning objects where a comfort to DS, and even if he had nothing to spin, he would just spin his hands round and round. He still does.
I decided to go with it and buy him any toys I could that involved some element of spinning, so he still had a variety of stimuli. It is a schema, and some children do get fixated on it, it's not always an indication of autistic spectrum disorder.
Buy him some poi (those spinning things with ribbon on them), spinning tops, all sorts of toy cars and vehicles (for the spinning wheels) toy washing machine, those salad spinners (one of DS's favourites), anythin in fact. I also found a tihng that you spin and add paint to and it makes nice patters on paper that DS loves doing (although your DS is a bit young for this now). yoyos, bits of string, glow sticks in the dark, loads of possibilities!
Sometimes when DS was younger and got stressed out or tired he needed to spin to comfort himself. When he's anxious or in new situations it helps calm him down.
He can sometimes go into a 'zone' when spinning and it's hard to draw his focus away, and I won't lie and say it has never frustrated, annoyed or worried me, because it has and it still does sometimes. It has also embarrassed me (once with a new boyfriend when he came down the stairs spinning one of my bras) and then there was the time he spun a carton of apple juice in a car full of prim aquaintances), and there was the time he spun a peice of pizza in a restaurant (toppings flew everywhere) but on the whole, it isn't a problem really.
Please don't worry, that's all I'm saying, and don't feel you have to stop him spinning, it won't do him any harm and can be put to good use.